| Instrument: | Meade 2080 | Celestron 11 |
| Type: | Schmidt Cassegrain | Schmidt Cassegrain |
| Clear aperture: | 203 mm | 280 mm |
| Focal length: | 2120 mm | 3080 mm |
| Focal ratio: | 1/10.6 | 1/11 |
Considering the moving primary mirror of our SCTs we decided to use off axis guiding to avoid problems with differential flexure and mirror flop. For the Meade 2080 we use a Lumicon Cassegrain Easy Guider with or without the focal reducing lens, which results in focal ratios of f/10.6 and f/6, respectively. For the C-11 we use the Lumicon Giant Easy Guider in its three modes of operation. Without reducing lens at 3080 mm (f/11), with the reducer in the rear position at 1960 mm (f/7) and with the reducer in the front position at 1705 mm (f/6.1).
For guiding we used until 1998 a Vixen GA-2 device, which allows the use of different eyepieces with or without a 2x Barlow lens. The GA-2 projects a bulls-eye reticle pattern into the field of view. Additionally that reticle can be rotated and shifted in two perpendicular directions, thus the guidestar need not be at the center of the field of view and you can always make the crosshairs match the directions of RA and declination.
Most of the time a 16 mm Erfle and a 18 mm Ortho in conjunction with the Barlow and a diagonal prism worked well for guiding. Finding suitable guidestars was sometimes a real pain, but in most situations acceptable guidestars (i.e. brighter than magnitude 8) could be found.
Drawing the field of view and the corresponding area for guidestars on a transparency, we did the guidestar search with a sky atlas at home. Noting the distance and position angle and using selfmade scale grids at the telescope and guider to put the guider to the predetermined position helped to save a lot of time for acquiring the desired guidestar.
In 1998 we acquired a SBIG ST-4 autoguider for doing the tedious job of guiding longtime exposures. We use a custom made extension tube of our own design to allow for easy focusing the CCD camera. It guarantees that we reach exactly the same focus position again after e.g. removing and reinserting the CCD camera for getting a dark frame. Using a scale grid on the ST-4 together with the grid on the guider allows to rotate the CCD head exactly to the position where the x-axis corresponds to RA and the y-axis corresponds to declination. No trial and error here!
Since 1999 we use SkyMap Pro 6 instead of the sky atlas to select the field of view, the optimum guidestar, its distance to the target and the position angle. The results are plotted to a 'finder chart'. All we have to do then at the telescope is to center the target, place the guider extension tube to the correct distance reading and rotate the guider to the correct position angle.
Following the advise of Philip Perkins and adjusting the angle of the guiding prism in the GEG improved the quality of the guidestars considerably. The ST-4 has no problems at all with the shape of the stars. Somehow more troublesome is the quality of Celestron's fork mount. The periodic error is 43 arc seconds and the stability is poor, working in windy conditions is nearly impossible.
In 1989 we acquired a custom made fork mount, which can be seen in the image of the C-11. The design is by Rudolf Pressberger, the crafting was done by Siegfried Müller (both University of Vienna, Austria). Tracking and stability are simply outstanding! The periodic error is a mere undetectable 3 arc seconds. Hitting the fork arm with your fist, the stars make one small move and return to their original positions within the fraction of a second, all that without discernible vibrations. The damping characteristics are great. Mounted on a permanent pier, we only could use it for one year. After moving we did not yet find an acceptable location to set it up again. At a weight of 110 kg it's no longer portable, hence we had to return to using the original Celestron fork.
For a long time we used Kodak Technical Pan 2415 film hypered by others. We had no access to the hypering process and the storage, hence we experienced all sorts of problems with those films. Uneven hypering, loss of sensitivity, mottling, etc.
After deciding to do our own hypering we did a thorough research of publications on that topic. Some references made us believe that hypering with pure hydrogen would be of advantage. Thus we started our own experiments and compared hypering with hydrogen to forming gas. To make a long story short: Hydrogen hypering results in higher sensibility, measured as density above fog for identical exposures, and is done in much shorter periods of time, i.e. 3 hours compared to 26 hours for forming gas.
Our hypering process is made up by the following steps:
Our series of tests was done by hypering pieces of film with increments of 1 hour, exposing them as contact copy of a grey scale step wedge from Kodak (0.15 density increments), tacking the pieces together and developing them together in one tank. Afterwards the densities of the single steps were measured with a densitometer, the base fog density subtracted and the resulting values plotted as function of step wedge density (equal to log exposure). The figure below shows those curves.
It's obvious that hydrogen hypering for 3 hours achieves the best results. Another advantage is the fast availability of hypered film. You need not plan ahead for days with your hypering and you need not store unnecessary huge amounts of hypered film.